1. Field of the Invention
This invention relates generally to mooring structures and in particular to a single point mooring structure. More particularly, the invention relates to a tower apparatus having an improved structure for connecting the vessel to the tower.
2. Description of the Prior Art
Mooring systems which include a rigid arm connected to a chain anchored buoy or similar compliant mooring point to allow for the independent motion of the vessel are well known. Tower structures have been proposed by which a vessel is connected to a fixed tower by complaint means which allows freedom of motion between the tower structure and the vessel while providing restoring force and movement of the vessel with respect to the tower and which prevents collision between the vessel and the tower under any combination of wind, waves and current.
A published patent application of the European Patent Office, Publication Number 0105976, published Apr. 25, 1984, (hereafter the Willem Van Heijst publication) describes a single point mooring tower structure. The Van Heijst publication describes a tower structure fixed to the floor of the body of water. The vessel is connected to the tower by means of a pair of rigid arms, each of which is hinged at its vessel end to the vessel, to allow independent pivoting about two axes which are nearly parallel to the horizontal or lateral axis of the tower.
The Van Heijst tower structure includes a turntable rotatable about a vertical axis. Each rigid arm is connected to the tower by means of a tension member. Each tension member is connected between a point along the lateral axis of the turntable and one of the rigid arms. Hinges are provided to allow the rigid arms to pivot about the lateral axis with respect to the tension members. Hinges are also provided to allow the tension arms to pivot about the lateral axis and the longitudinal axis with respect to the turntable. Ballast weights are connected to the tension members.
While the Van Heijst structure has advantages over other prior art tower mooring structures, improvements in its design are desirable. For example, roll motions of the vessel will cause a twisting effect at the hinges connecting the rigid arms to the tension members which cannot be safely accommodated without significant modifications to the hinge arrangements. Also the Van Heijst structure requires two separate yoke frames, each with two large capacity hinges at the tanker end. A single yoke frame, as in the present invention, requires only two hinges at the tanker end resulting in both initial cost savings and reduced maintenance requirements.